On Tue, May 05, 2009 at 03:17:34PM +0100, Ian Jackson wrote:
>
> Consider a raw disk image file which is writeable by a guest. (This
> is of course one very common usage model.) The guest can write
> anything it likes to the image file, including anything to the start
> of the file - where the cow header would be if it were a cow file.
>
> So it can, if it likes, write a cow header (qcow2 for example) to the
> start of its `virtual disk image'. Qemu's cow headers contain the
> pathname of the backing file, and the guest can of course name any
> file it likes.
>
> If this image, which is supposedly a raw image, is then opened by any
> tool which autoguesses the format, that tool will then spot the cow
> header written by the guest and access the backing file (in the
> context of the host) specified by the guest.
>
> Depending on the exact circumstances this can allow the guest to get
> copies of or even complete read access to any data of its choice in
> the host.
>
> Upstream qemu have fixed this problem in a half-hearted way and
> evidently their qemu-img is still vulnerable. We have changed the
> format-determination code in block.c so that any attempt to autodetect
> a format never returns `raw'; that means that any vulnerable code
> anywhere is instantly fixed although it may break some existing usages
> in cases where we haven't properly plumbed through a specification of
> the image format.
Wasn't the upstream change to add a '-F baseimage_format' enough to
allow the flaw to be avoided when creating new images ? Or are you
attempting to prevent the issue, even when -F is not used ?
Daniel
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